The production of alkylene oxide, such as ethylene oxide, by the reaction of oxygen or oxygen-containing gases with ethylene in the presence of a silver-containing catalyst at elevated temperature is an old and well-known art. For example, U.S. Pat. No. 2,040,782, dated May 12, 1936, describes the manufacture of ethylene oxide by the reaction of oxygen with ethylene in the presence of silver catalysts which contain a class of metal-containing solid promoters. In Reissue U.S. Pat. 20,370, dated May 18, 1937, Leforte discloses that the formation of olefin oxides may be effected by causing olefins to combine directly with molecular oxygen in the presence of a silver catalyst. (An excellent discussion on ethylene oxide, including a detailed description of commonly used manufacturing process steps, is found in Kirk-Othmer's Encyclopedia of Chemical Technology, 4th Ed.(1994) Volume 9, pages 915 to 959).
The catalyst is the most important element in direct oxidation of ethylene to produce ethylene oxide. There are several well-known essential components of such catalyst: silver; a suitable support/carrier (for example alpha-alumina); and promoters, all of which can play a role in improving catalyst performance. Because of the importance of the catalyst in the production of ethylene oxide, much effort has been expended to improve the performance of such catalysts.
The use of suitable promoters is an effective and proven way to enhance the performance of the catalyst in the production of alkylene oxide, for example ethylene oxide, and such use is well known to those skilled in the art. There are at least two types of promoters—solid promoters and gaseous promoters. A solid promoter is incorporated into the catalyst prior to its use, either as a part of the carrier (that is support) or as a part of the silver component applied thereto. When a solid promoter is added during the preparation of the catalyst, the promoter may be added to the carrier before the silver component is deposited thereon, added simultaneously with the silver component, or added sequentially following the deposition of the silver component on the carrier. Examples of well-known solid promoters for catalysts used to produce ethylene oxide include compounds of potassium, rubidium, cesium, rhenium, sulfur, manganese, molybdenum, and tungsten. During the use of the catalyst in the reaction to make ethylene oxide, the specific form of the promoter in the catalyst may be unknown.
In contrast, the gaseous promoters are gas-phase compounds and or mixtures thereof which are introduced to a reactor for the production of alkylene oxide (for example ethylene oxide) with vapor-phase reactants, such as ethylene and oxygen. Such promoters further enhance the performance of a given catalyst, working in conjunction with or in addition to the solid promoters.
It is well known that for catalysts using certain solid promoters, in particular those employing at least one efficiency-enhancing salt of a member of a redox-half reaction pair, the addition of a gaseous component capable of producing a member of a redox-half reaction pair is advantageous for maintaining selectivity and activity (see U.S. Pat. Nos. 5,387,751, 4,837,194, 4,831,162, 4,994,587, 4,994,589, 4,994,588, 5,504,053, 5,187,140, and 6,511,938 B1).
As used herein, the term “salt” does not require that the cation and anion components of the salt be associated with or bonded to one another in the solid catalyst, but rather implies that both components are present in some form in the catalyst under reaction conditions.
In the catalysts used in the process of the present invention, the gaseous component capable of producing a member of a redox-half reaction pair under reaction conditions is a nitrogen-containing gas, such as for example nitric oxide, nitrogen dioxide and/or dinitrogen tetroxide, hydrazine, hydroxylamine or ammonia, nitroparaffins (for example, nitromethane), nitroaromatic compounds (for example, nitrobenzene), N-nitro compounds, and/or nitriles (for example, acetonitrile). The amount of gaseous nitrogen-containing promoter to be used in these catalysts is that amount sufficient to enhance the performance, such as the activity of the catalyst and particularly the efficiency of the catalyst. The amount of gaseous nitrogen-containing promoter is generally described in the aforementioned patents as being determined by the particular efficiency-enhancing salt of a member of a redox-half reaction pair used and the concentration thereof, the particular alkene undergoing oxidation, and by other factors including the amount of carbon dioxide in the inlet reaction gases. For example, U.S. Pat. No. 5,504,053 discloses that when the gaseous nitrogen-containing promoter is NO (nitric oxide), a suitable concentration is from 0.1 to 100 ppm, by volume, of the gas stream. Preferably, when CO2 (carbon dioxide) is present in amounts up to 3 volume percent, the NO is present in 0.1 to 60 ppmv, preferably 1 to 40 ppmv. Similarly, U.S. Pat. No. 5,387,751 discloses a continuous process for making ethylene oxide which comprises contacting ethylene, oxygen, a silver-containing catalyst and from 1 to 50 parts per million by weight of vinyl chloride reaction modifier with a nitrogen oxide in a concentration of 0.5 to 50 ppm of NO2 equivalent of the process gas by volume, said nitrogen oxide forming nitrate and or nitrite ions in the catalyst under process conditions.
It is known in the catalyst literature that the concentration of gaseous chlorine-containing promoter (also referred to as modifier or inhibitor) which is required for optimum catalyst performance is dependent on the amounts of hydrocarbons present in the gas phase and other factors (J. M. Berty, Applied Industrial Catalysis, Vol. 1, Chapter 8, p. 224-227, 1983). The specific reactions by which hydrocarbons remove chloride from the catalyst surface are cited, and while ethane is reported to strip chlorides very effectively, ethylene is also capable of removing chlorides but is found to be significantly less effective. WO 03/044002 A1 and WO 03/044003 A1 disclose methods for optimizing gaseous promoters for high selectivity catalysts in reaction phases with differing feed compositions and temperatures, respectively. The optimum modifier level is proposed to depend on the effective molar quantity of the hydrocarbon present in the feed and the effective molar quantity of the active species of the reaction modifier.
Several terms are commonly used to describe some of the parameters of catalytic systems for epoxidation of alkenes. For instance, “conversion” is defined as the molar percentage of alkene fed to the reactor which undergoes reaction. Of the total amount of alkene which is converted to a different chemical entity in a reaction process, the molar percentage which is converted to the corresponding alkylene epoxide is known as the “efficiency” (which is synonymous with the “selectivity”) of that process. The product of the percent efficiency times the percent conversion (divided by 100 percent to convert from percent2 to percent) is the percentage “yield”, that is, the molar percentage of the alkene fed that is converted into the corresponding epoxide.
The “activity” of a catalyst can be quantified in a number of ways, one being the mole percent of alkylene epoxide contained in the outlet stream of the reactor relative to that in the inlet stream (the mole percent of alkylene epoxide in the inlet stream is typically, but not necessarily, zero percent) while the reactor temperature is maintained substantially constant, and another being the temperature required to maintain a given rate of alkylene epoxide production. That is, in many instances, activity is measured over a period of time in terms of the molar percent of alkylene epoxide produced at a specified constant temperature. Alternatively, activity may be measured as a function of the temperature required to sustain production of a specified constant mole percent of alkylene epoxide. The useful life of a catalysts system is the length of time that reactants can be passed through the reaction system during which results are obtained which are considered by the operator to be acceptable in light of all relevant factors.
“Deactivation”, as used herein, refers to a permanent loss of activity and/or efficiency, that is, a decrease in activity and/or efficiency which cannot be recovered. As noted above, production of alkylene epoxide product can be increased by raising the temperature, but the need to operate at a higher temperature to maintain a particular rate of production is representative of activity deactivation. The “stability” of a catalyst is inversely proportional to the rate of deactivation, that is, the rate of decrease of efficiency and/or activity. Lower rates of decline of efficiency and/or activity are generally desirable.
To be considered satisfactory, a catalyst must have acceptable activity and efficiency, and the catalyst must also have sufficient stability, so that it will have a sufficiently long useful life. When the efficiency and/or activity of a catalyst has declined to an unacceptably low level, typically the reactor must be shut down and partially dismantled to remove the catalyst. This results in losses in time, productivity and materials, for example, silver catalytic material and alumina carrier. In addition, the catalyst must be replaced and the silver salvaged or, where possible, regenerated. Even when a catalyst is capable of regeneration in situ, generally production must be halted for some period of time. At best, replacement or regeneration of catalyst requires additional losses in production time to treat the catalyst and, at worst, requires replacement of the catalyst with the associated costs. It is therefore highly desirable to find ways to lengthen the useful life of a catalyst.